arXiv subjects

C. Liu

Publications and source records attributed to C. Liu.

At least 37 records · Page 2Linked to original sources

arXiv2025

ZTF SN Ia DR2: High-velocity components in the Si II $λ$6355

The ZTF SN Ia Data Release 2 provides a perfect opportunity to perform a thorough search for, and subsequent analysis of, high-velocity components in the Si II $λ$6355 feature in the pre-peak regime. The source of such features remains unclear, with potential origins in circumstellar material or density/abundance enhancements intrinsic to the SN ejecta. Therefore, they may provide clues to the elusive progenitor and explosion scenarios of SNe Ia. We employ a MCMC fitting method followed by BIC testing to classify single and double Si II $λ$6355 components in the DR2. The detection efficiency of our classification method is investigated through the fitting of simulated features, allowing us to place cuts upon spectral quality required for reliable classification. These simulations were also used to perform an analysis of the recovered parameter uncertainties and potential biases in the measurements. Within the 329 spectra sample that we investigate, we identify 85 spectra exhibiting Si II $λ$6355 HVFs. We find that HVFs decrease in strength with phase relative to their photospheric counterparts - however, this decrease can occur at different phases for different objects. HVFs with larger velocity separations from the photosphere are seen to fade earlier leaving only the double components with smaller separations as we move towards maximum light. Our findings suggest that around three quarters of SN Ia spectra before -11 d show high-velocity components in the Si II $λ$6355 with this dropping to around one third in the six days before maximum light. We observe no difference between the populations of SNe Ia that do and do not form Si II $λ$6355 HVFs in terms of SALT2 light-curve parameter x1, peak magnitude, decline rate, host mass, or host colour, supporting the idea that these features are ubiquitous across the SN Ia population.

astro-ph.HE↗

arXiv2025

ZTF SN Ia DR2: The diversity and relative rates of the thermonuclear SN population

The Zwicky Transient Facility SN Ia Data Release 2 (ZTF SN Ia DR2) contains more than 3,000 Type Ia supernovae (SNe Ia), providing the largest homogeneous low-redshift sample of SNe Ia. Having at least one spectrum per event, this data collection is ideal for large-scale statistical studies of the photometric, spectroscopic and host-galaxy properties of SNe Ia, particularly of the rarer 'peculiar' sub-classes. In this paper we first present the method we developed to spectroscopically classify the SNe in the sample, and the techniques we used to model their multi-band light curves and explore their photometric properties. We then show a method to distinguish between the peculiar sub-types and the normal SNe Ia. We also explore the properties of their host galaxies and estimate their relative rates, focusing on the peculiar sub-types and their connection to the cosmologically useful SNe Ia. Finally, we discuss the implications of our study with respect to the progenitor systems of the peculiar SN Ia events.

astro-ph.HE↗

arXiv2024

Influences of $δ$B Contribution and Parallel Inertial Term of Energetic Particles on MHD-Kinetic Hybrid Simulations: A Case Study of the 1/1 Internal Kink Mode

The magnetohydrodynamic-kinetic (MHD-kinetic) hybrid model [Park et. al., 1992] has been widely applied in studying energetic particles (EPs) problems in fusion plasmas for past decades. The pressure-coupling scheme or the current-coupling scheme is adopted in this model. However, two noteworthy issues arise in the model application: firstly, the coupled term introduced in the pressure-coupling scheme, $\left( \nabla \cdot \mathbf{P}_{\mathrm{h}} \right)_{\bot}$, is often simplified by $\nabla \cdot \mathbf{P}_{\mathrm{h}}$, which is equivalent to neglecting the parallel inertial term of EPs; secondly, besides the $δf $ contribution caused by changing in the EP distribution function, the magnetic field perturbation (the $δ\mathbf{B} $ contribution) generated during development of the instabilities should also be considered, but it is often ignored in existing hybrid simulations. In this paper, we derive the analytical formulations under these two coupling schemes and then numerically study the representative case of the linear stability of the m/n=1/1 internal kink mode (IKM) [Fu et. al., 2006] by using the CLT-K code. It is found that the approximated models can still yield reasonable results when EPs are isotopically distributed. But it fails completely in cases with anisotropic EP distributions. In addition, we further investigate the influence of EP's orbit width on the stability of IKM and verify the equivalence between pressure-coupling scheme and the current-coupling scheme.

physics.plasm-ph↗

arXiv2024

The Massive and Quiescent Elliptical Host Galaxy of the Repeating Fast Radio Burst FRB20240209A

The discovery and localization of FRB20240209A by the Canadian Hydrogen Intensity Mapping Fast Radio Burst (CHIME/FRB) experiment marks the first repeating FRB localized with the CHIME/FRB Outriggers and adds to the small sample of repeating FRBs with associated host galaxies. Here we present Keck and Gemini observations of the host that reveal a redshift $z=0.1384\pm0.0004$. We perform stellar population modeling to jointly fit the optical through mid-infrared data of the host and infer a median stellar mass log$(M_*/{\rm M_{\odot}})=11.34\pm0.01$ and a mass-weighted stellar population age $\sim11$Gyr, corresponding to the most massive and oldest FRB host discovered to date. Coupled with a star formation rate $<0.36\,{\rm M_{\odot}\ yr^{-1}}$, the specific star formation rate $<10^{-11.8}\rm\ yr^{-1}$ classifies the host as quiescent. Through surface brightness profile modeling, we determine an elliptical galaxy morphology, marking the host as the first confirmed elliptical FRB host. The discovery of a quiescent early-type host galaxy within a transient class predominantly characterized by late-type star-forming hosts is reminiscent of short-duration gamma-ray bursts, Type Ia supernovae, and ultraluminous X-ray sources. Based on these shared host demographics, coupled with a large offset as demonstrated in our companion paper, we conclude that preferred progenitors for FRB20240209A include magnetars formed through merging binary neutron stars/white dwarfs or the accretion-induced collapse of a white dwarf, or a luminous X-ray binary. Together with FRB20200120E localized to a globular cluster in M81, our findings provide strong evidence that some fraction of FRBs may arise from a process distinct from the core collapse of massive stars.

astro-ph.HE↗

arXiv2024

An Integrated Framework for Uncertainty Quantification in High Temperature Gas Cooled Reactors using the HCP Time-dependent Multiphysics code and Dakota toolkit

The High Temperature Reactor Code Package provides sophisticated modeling and simulation capabilities for high temperature gas cooled reactors like the HTR-200 Modul. However, HCP currently lacks integrated methods for uncertainty quantification and sensitivity analysis. This work aims to couple HCP with the DAKOTA toolkit to enable UQ workflows for quantifying how different uncertainties impact HTGR system performance. DAKOTA offers state of the art sampling and analysis methods that will be linked to the HCP time-dependent multiphysics environment. Key input parameters related to manufacturing variability, boundary and initial conditions, and material properties will be defined as uncertain in this study. Both steady state and time-dependent multiphysics simulations will be analyzed to understand the relative importance of uncertainties across different physics phenomena. Output metrics of interest can include anything measured by the HCP code. Results show the HTR-200 Modul design's robustness to input uncertainties related to inlet gas temperature, U-235 enrichment, graphite density, inlet mass flow rate, and reactor power. A pressurized loss of forced cooling transient was simulated against uncertainty in the inlet gas temperature. Results show the maximum fuel temperature is within design limits with a relatively big safety margin even when considering uncertainty in the boundary conditions of the reactor. Future work will expand the analysis to more HCP physics modules and nuclear data uncertainties. By leveraging UQ and sensitivity analysis with the linked HCP/DAKOTA environment, this work will provide valuable new insights into the expected performance variability of HTR systems during normal and off-normal conditions. The framework developed here will aid uncertainty management activities by gas-cooled reactor developers, researchers, and regulators.

physics.ins-det↗

arXiv2024

PointNetPGAP-SLC: A 3D LiDAR-based Place Recognition Approach with Segment-level Consistency Training for Mobile Robots in Horticulture

3D LiDAR-based place recognition remains largely underexplored in horticultural environments, which present unique challenges due to their semi-permeable nature to laser beams. This characteristic often results in highly similar LiDAR scans from adjacent rows, leading to descriptor ambiguity and, consequently, compromised retrieval performance. In this work, we address the challenges of 3D LiDAR place recognition in horticultural environments, particularly focusing on inter-row ambiguity by introducing three key contributions: (i) a novel model, PointNetPGAP, which combines the outputs of two statistically-inspired aggregators into a single descriptor; (ii) a Segment-Level Consistency (SLC) model, used exclusively during training to enhance descriptor robustness; and (iii) the HORTO-3DLM dataset, comprising LiDAR sequences from orchards and strawberry fields. Experimental evaluations conducted on the HORTO-3DLM and KITTI Odometry datasets demonstrate that PointNetPGAP outperforms state-of-the-art models, including OverlapTransformer and PointNetVLAD, particularly when the SLC model is applied. These results underscore the model's superiority, especially in horticultural environments, by significantly improving retrieval performance in segments with higher ambiguity.

cs.RO↗

arXiv2024

Spinons in a new Shastry-Sutherland lattice magnet Pr$_2$Ga$_2$BeO$_7$

Identifying the elusive spinon excitations in quantum spin liquid (QSL) materials is what scientists have long sought for. Recently, thermal conductivity ($κ$) has emerged to be a decisive probe because the fermionic nature of spinons leads to a characteristic nonzero linear $κ_0/T$ term while approaching zero Kelvin. So far, only a few systems have been reported to exhibit such term. Here, we report a $κ_0/T \approx$ 0.01 WK$^{-2}$m$^{-1}$, the largest $κ_0/T$ value ever observed in magnetic oxide QSL candidates, in a new quantum magnet Pr$_2$Ga$_2$BeO$_7$ with a Shastry-Sutherland lattice (SSL). Its QSL nature is further supported by the power-law temperature dependence of the specific heat, a plateau of muon spin relaxation rate, and gapless inelastic neutron spectra. Our theoretical analysis reveals that the introduction of XY spin anisotropy is the key for Pr$_2$Ga$_2$BeO$_7$ to be the first QSL realized on the SSL, after more than four decades of extensive studies on this celebrated magnetically frustrated lattice.

cond-mat.str-el↗

arXiv2024

Direct RF Sampling based LLRF Control System for C-band Linear Accelerator

Low Level RF (LLRF) control systems of linear accelerators (LINACs) are typically implemented with heterodyne based architectures, which have complex analog RF mixers for up and down conversion. The Gen 3 Radio Frequency System-on-Chip (RFSoC) device from AMD Xilinx integrates data converters with maximum RF frequency of 6~GHz. This enables direct RF sampling of C-band LLRF signal typically operated at 5.712 GHz without any analogue mixers, which can significantly simplify the system architecture. The data converters sample RF signals in higher order Nyquist zones and then up or down convert digitally by the integrated data path in RFSoC. The closed-loop feedback control firmware implemented in FPGA integrated in RFSoC can process the base-band signal from the ADC data path and calculate the updated phase and amplitude to be up-mixed by the DAC data path. We have developed a C-band LLRF control RFSoC platform with direct RF sampling, which targets Cool Copper Collider (C3) and other C or S band LINAC research and development projects. In this paper, the architecture of the platform will be described. We have optimized the configuration of the data converter and characterized performance of them with RF pulses. The test results for some of the key performance parameters for the LLRF platform with our custom solid-state amplifier, such as phase and amplitude stability, will be discussed in this paper.

physics.acc-ph↗

arXiv2024

Characteristics of the MTx optical transmitter in Total Ionizing Dose

The dual-channel multi-mode 850 nm optical Miniature Transmitter (MTx) is developed for data transmission of the ATLAS LAr calorimeter readout at LHC. The MTx's are exposed to the radiation field of proton-proton collisions, therefore, the tolerance in Total Ionizing Dose (TID) is required. The TID effects in the MTx are investigated with X-rays and Co-60 gamma-rays for the active components of VCSEL diodes and the customized Link-on-Chip laser driver (LOCld) developed in 0.25 um Silicon-on-Sapphire CMOS technology. The irradiation tests were conducted at various dose rates. The responses to TID are observed with degradation of laser currents at initial dose of 10 to 100 Gy(SiO2), and partial recovery with additional TID to a stable output about 90 % of the original. The optical eye diagrams of irradiated samples show slightly increased jittering, and are suitable for the ATLAS requirement of 5 Gbps applications.

physics.ins-det↗

arXiv2024

A Configurable Ultra-Low Noise Current Source for Transition-Edge Sensor Characterization

Transition-edge sensors (TESs) are sensitive devices for detecting photons from millimeter radiation to gamma rays. Their photon counting efficiency and collecting area benefit from large-array multiplexing scheme, and therefore the development of multiplexing readout system has been an important topic in this field. Among the many multiplex techniques, time-division multiplexing (TDM) superconducting quantum interference device (SQUID) has been used most widely for TES readout. In this work, we design a Configurable Ultra-Low Noise Current Source (CLCS) for TES characterization and as a part of a whole TDM-TES bias control system. The CLCS is based on the feedback structure of ultra-low noise instrumentation amplifiers and low-noise, high-resolution (20 bits) digital-to-analog converter (DAC). CLCS has an ultra-high resolution of 10 nA in the 0 to 5 mA current output range, and can perform current-voltage (IV) sweep and bias-step tests to measure key TES parameters on board. The feedback structure of the CLCS also avoids the issue of impedance mismatch.

astro-ph.IM↗

arXiv2024

Stellar halo density with LAMOST K and M giants

AIMS. We derive the morphology of the stellar component in the outer halo volume, and search for possible overdensities due to substructures therein. METHODS. We made use of some of the data releases of the spectroscopic survey LAMOST DR8-DR9 in tandem with distance determinations for two subsamples, that is, of K-giants and M-giants, respectively, making up 60,000 stars. These distance are obtained through Bayesian techniques that derive absolute magnitudes as a function of measured spectroscopic parameters. Our calculation of the density from these catalogues requires: (1) derivation of the selection function; and (2) a correction for the convolution of the distance errors, which we carried out with Lucy's inversion of the corresponding integral equation. RESULTS. The stellar density distribution of the outer halo (distance to the Galactic centre, $r_G$, of between 25 and 90 kpc) is a smooth monotonously decreasing function with a dependence of approximately $ρ\propto r_G^{-n}$, with $n=4.6\pm 0.4$ for K-giants and $n=4.5\pm 0.2$ for M-giants, and with a insignificant oblateness. The value of $n$ is independent of the angular distance to the Sagittarius tidal stream plane, which is what would be expected if such a stream did not exist in the anticenter positions or had a negligible imprint in the density distribution in the outer halo. Apart from random fluctuations or minor anomalies in some lines of sight, we do not see substructures superimposed in the outer halo volume within the resolution that we are using and limited by the error bars. This constrains the mass of over- and under-densities in the outer halo to be of $\lesssim 10^3$ M$_\odot $/deg$^2$, whereas the total mass of the stellar halo, including inner and outer parts, is $\sim 7\times 10^8$ M$_\odot $.

astro-ph.GA↗

arXiv2024

ORCHNet: A Robust Global Feature Aggregation approach for 3D LiDAR-based Place recognition in Orchards

Robust and reliable place recognition and loop closure detection in agricultural environments is still an open problem. In particular, orchards are a difficult case study due to structural similarity across the entire field. In this work, we address the place recognition problem in orchards resorting to 3D LiDAR data, which is considered a key modality for robustness. Hence, we propose ORCHNet, a deep-learning-based approach that maps 3D-LiDAR scans to global descriptors. Specifically, this work proposes a new global feature aggregation approach, which fuses multiple aggregation methods into a robust global descriptor. ORCHNet is evaluated on real-world data collected in orchards, comprising data from the summer and autumn seasons. To assess the robustness, we compare ORCHNet with state-of-the-art aggregation approaches on data from the same season and across seasons. Moreover, we additionally evaluate the proposed approach as part of a localization framework, where ORCHNet is used as a loop closure detector. The empirical results indicate that, on the place recognition task, ORCHNet outperforms the remaining approaches, and is also more robust across seasons. As for the localization, the edge cases where the path goes through the trees are solved when integrating ORCHNet as a loop detector, showing the potential applicability of the proposed approach in this task. The code will be publicly available at:\url{https://github.com/Cybonic/ORCHNet.git}

cs.CV↗

arXiv2024

Saturation of fishbone instability through zonal flows driven by energetic particle transport in tokamak plasmas

Gyrokinetic and kinetic-MHD simulations are performed for the fishbone instability in the DIII-D discharge #178631, chosen for validation of first-principles simulations to predict the energetic particle (EP) transport in an ITER prefusion baseline scenario. Fishbone modes are found to generate zonal flows, which dominate the fishbone saturation. The underlying mechanisms of the two-way fishbone-zonal flows nonlinear interplay are discussed in details. Numerical and analytical analyses identify the fishbone-induced EP redistribution as the dominant generation mechanism for zonal flows. The zonal flows modify the nonlinear dynamics of phase space zonal structures, which reduces the amount of EPs able to resonate with the mode, leading to an early fishbone saturation. Simulation results including zonal flows agree quantitatively with DIII-D experimental measurements of the fishbone saturation amplitude and EP transport, supporting this novel saturation mechanism by self-generated zonal flows. Moreover, the wave-particle mode-locking mechanism is shown to determine quantitatively the fishbone frequency down-chirping, as evident in GTC simulation results in agreement with predictions from analytical theory. Finally, the fishbone-induced zonal flows are possibly responsible for the formation of an ion-ITB in the DIII-D discharge. Based on the low EP transport and the large zonal flow shearing rates associated with the fishbone instability in gyrokinetic simulations of the ITER scenario, it is conjectured that high performance scenarios could be designed in ITER burning plasmas through fishbone-induced ITBs.

physics.plasm-ph↗

arXiv2024

Saturation of fishbone instability by self-generated zonal flows in tokamak plasmas

Gyrokinetic simulations of the fishbone instability in DIII-D tokamak plasmas find that self-generated zonal flows can dominate the nonlinear saturation by preventing coherent structures from persisting or drifting in the energetic particle phase space when the mode frequency down-chirps. Results from the simulation with zonal flows agree quantitatively, for the first time, with experimental measurements of the fishbone saturation amplitude and energetic particle transport. Moreover, the fishbone-induced zonal flows are likely responsible for the formation of an internal transport barrier that was observed after fishbone bursts in this DIII-D experiment. Finally, gyrokinetic simulations of a related ITER baseline scenario show that the fishbone induces insignificant energetic particle redistribution and may enable high performance scenarios in ITER burning plasma experiments.

physics.plasm-ph↗

arXiv2023

LLRF System Considerations for a Compact, Commercial C-band Accelerator using the AMD Xilinx RF-SoC

This work describes the LLRF and control system in use for a novel accelerator structure developed for a compact design operating in C-band developed by SLAC, with collaboration from RadiaBeam and RadiaSoft. This design is a pulsed RF/pulsed beam system that only provides minimal monitoring for control of each two-cavity pair. Available signals include only a forward and reflected signal for each pair; such a design requires careful consideration of calibration and power-on routines, as well an understanding of how to correct for disturbances caused by the entire RF signal chain, including a new SSA, klystron, and distribution system. An AMD Xilinx RF-SoC with a separate supervisory computer is the LLRF system core, with on-board pulse-to-pulse feedback corrections. This work presents the current status of the project, as well as obstacles and manufacturing plans from the viewpoint of developing for larger-volume manufacturing. This material is based upon work supported by the Defense Advanced Research Projects Agency under Contract Numbers 140D0423C0006 and 140D0423C0007. The views, opinions, and/or findings expressed are those of the author(s) and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.

physics.acc-ph↗

arXiv2023

Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843$-$033 Region

HESS J1843$-$033 is a very-high-energy gamma-ray source whose origin remains unidentified. This work presents, for the first time, the energy spectrum of gamma rays beyond $100\, {\rm TeV}$ from the HESS J1843$-$033 region using the data recorded by the Tibet air shower array and its underground muon detector array. A gamma-ray source with an extension of $0.34^{\circ} \pm 0.12^{\circ}$ is successfully detected above $25\, {\rm TeV}$ at $(α,\, δ) = (281.09^{\circ}\pm 0.10^{\circ},\, -3.76^{\circ}\pm 0.09^{\circ})$ near HESS J1843$-$033 with a statistical significance of $6.2\, σ$, and the source is named TASG J1844$-$038. The position of TASG J1844$-$038 is consistent with those of HESS J1843$-$033, eHWC J1842$-$035, and LHAASO J1843$-$0338. The measured gamma-ray energy spectrum in $25\, {\rm TeV} < E < 130\, {\rm TeV}$ is described with ${\rm d}N/{\rm d}E = (9.70\pm 1.89)\times 10^{-16} (E/40\, {\rm TeV})^{-3.26\pm 0.30}\, {\rm TeV}^{-1} {\rm cm}^{-2} {\rm s}^{-1}$, and the spectral fit to the combined spectra of HESS J1843$-$033, LHAASO J1843$-$0338, and TASG J1844$-$038 implies the existence of a cutoff at $49.5\pm 9.0\, {\rm TeV}$. Associations of TASG J1844-038 with SNR G28.6$-$0.1 and PSR J1844-0346 are also discussed in detail for the first time.

astro-ph.HE↗

arXiv2023

Observation of gamma rays up to 320 TeV from the middle-aged TeV pulsar wind nebula HESS J1849$-$000

Gamma rays from HESS J1849$-$000, a middle-aged TeV pulsar wind nebula (PWN), are observed by the Tibet air shower array and the muon detector array. The detection significance of gamma rays reaches $4.0\, σ$ and $4.4\, σ$ levels above 25 TeV and 100 TeV, respectively, in units of Gaussian standard deviation $σ$. The energy spectrum measured between $40\, {\rm TeV} < E < 320\, {\rm TeV}$ for the first time is described with a simple power-law function of ${\rm d}N/{\rm d}E = (2.86 \pm 1.44) \times 10^{-16}(E/40\, {\rm TeV})^{-2.24 \pm 0.41}\, {\rm TeV}^{-1}\, {\rm cm}^{-2}\, {\rm s}^{-1}$. The gamma-ray energy spectrum from the sub-TeV ($E < 1\, {\rm TeV}$) to sub-PeV ($100\, {\rm TeV} < E < 1\, {\rm PeV}$) ranges including the results of previous studies can be modeled with the leptonic scenario, inverse Compton scattering by high-energy electrons accelerated by the PWN of PSR J1849$-$0001. On the other hand, the gamma-ray energy spectrum can also be modeled with the hadronic scenario in which gamma rays are generated from the decay of neutral pions produced by collisions between accelerated cosmic-ray protons and the ambient molecular cloud found in the gamma-ray emitting region. The cutoff energy of cosmic-ray protons $E_{\rm p\, cut}$, cut is estimated at ${\rm log}_{10}(E_{\rm p,\, cut}/{\rm TeV}) = 3.73^{+2.98}_{-0.66}$, suggesting that protons are accelerated up to the PeV energy range. Our study thus proposes that HESS J1849$-$000 should be further investigated as a new candidate for a Galactic PeV cosmic-ray accelerator, PeVatron.

astro-ph.HE↗

arXiv2023

Exploring asymmetric substructures of the outer disk based on the conjugate angle of the radial action

We use the conjugate angle of radial action ($θ_R$), the best representation of the orbital phase, to explore the "mid-plane, north branch, south branch" and "Monoceros area" disk structures that were previously revealed in the LAMOST K giants (Xu et al. 2020). The former three substructures, identified by their 3D kinematical distributions, have been shown to be projections of the phase space spiral (resulting from nonequilibrium phase mixing). In this work, we find that all of these substructures associated with the phase spiral show high aggregation in conjugate angle phase space, indicating that the clumping in conjugate angle space is a feature of ongoing, incomplete phase mixing. We do not find the $Z-V_Z$ phase spiral located in the "Monoceros area", but we do find a very highly concentrated substructure in the quadrant of conjugate angle space with the orbital phase from the apocenter to the guiding radius. The existence of the clump in conjugate angle space provides a complementary way to connect the "Monoceros area" with the direct response to a perturbation from a significant gravitationally interactive event. Using test particle simulations, we show that these features are analogous to disturbances caused by the impact of the last passage of the Sagittarius dwarf spheroidal galaxy.

astro-ph.GA↗